The invention relates to amorphous semiconductor films or other bodies with uniquely low defect states in the energy gap, such as dangling bonds, recombination centers, etc., to provide improved amorphous semiconductor films which have characteristics like those found in corresponding crystalline semiconductors. By the term "amorphous" is meant a material which has long range disorder, although it may have an ordered structure in the short or intermediate range order, or it may contain isolate domains having an ordered structure in a primarily amorphous matrix. The amorphous films involved have one of their most important applications in photovoltaic devices, and current control devices, such as various p-n junction devices such as p-i-n and m-i-s, devices, rectifiers, transistors or the like, where heretofore crystalline semiconductor bodies have been used in their fabrication.
The principles involved in the invention have their most important and useful application to amorphous silicon and silicon-alloy bodies, especially thin films of such materials, although many aspects thereof are also applicable to various other similar amorphous semiconductor bodies, and formed by elements including individual elements or mixtures or alloys of elements which have localized defect states in the energy gap which adversely affect certain desired electrical characteristics thereof.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency crystalline solar cells. Since amorphous silicon-containing films, if made equivalent to crystalline silicon films, would have many advantages over such crystalline silicon films (e.g. lower cost, larger area, easier and faster manufacture), the main purpose of this invention is to provide amorphous semiconductor bodies which have characteristics closely resembling those of corresponding crystalline materials.
When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially impurity-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p and n conductivity regions therein. This was accomplished by diffusing into such pure crystalline materials parts per million (i.e. less than 1 percent) of donor or acceptor dopant materials introduced as substantial impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction and photoconductive crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants. These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material has all resulted in a serious economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Fabricating transistor-like p-n junction, photoconductive or solar cell devices from polycrystalline materials has also been found unsatisfactory for reasons including those applicable to devices made from single crystalline materials, as well as adding defect states and the problems caused by such things as grain boundaries.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p and n junctions. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films were found to have microvoids and dangling bonds and other defects which produce a high density of localized undesired defect states in the energy gap thereof. The presence of a high density of localized defect states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short diffusion lengths, making such films unsuitable for solar cell applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction of valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975. As there reported an amorphous silicon film was formed by glow discharge deposition of silane (SiH.sub.4) gas in the presence of a doping gas of phosphine (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.6 H.sub.6) for p-type conduction. These gases were premixed and passed through a reaction tube where the gaseous mixture was decomposed by an r.f. glow discharge and deposited on a substrate at a high substrate temperature of about 350.degree.-600.degree. K. The material so deposited on the substrate was an amorphous silicon host matrix material where the phosphorous or boron formed dopants in the silicon host matrix material in concentrations between about 5.times.10.sup.-6 and 10 .sup.-2 parts per volume. When the localized defect states in the energy gap of the undoped form of these materials were measured it was found that the density of localized defect states in the energy gap thereof was substantially reduced from that previously measured for other amorphous silicon films deposited by other processes. However, a much more substantial reduction in the minimum density of these localized defect states was necessary to bring the electrical characteristics of amorphous silicon materials much closer to those of corresponding crystalline materials.
It was not originally known by these researchers that the silicon films they deposited were, instead of relatively pure amorphous silicon films, an alloy of silicon and hydrogen and that the hydrogen alloyed with the silicon to eliminate many of the localized defect states.
Thus, after this initial development of the glow discharge deposition of silicon from silane gas was carried out, work was done by others on the sputter depositing of amorphous silicon films in an atmosphere of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as a compensating agent to reduce the localized defect states of the energy gap. However, the degree of reduction in the density of localized defect states achieved by this sputter deposition process was less than that achieved by the silane deposition process described above (as would be expected since sputter and evaporation deposition processes inherently produce amorphous films with much higher densities of localized states than does a glow discharge deposition process). This is the reason that it was not believed that sputter or evaporation deposition processes could successfully produce amorphous semiconductor films functionally equivalent to similar crystalline materials used in solar cell and current control devices. Also, the sputtering process must be carried out under certain critical partial pressure limitations, and since such partial pressures are effected both by the amount of argon and hydrogen gas present, the amount of molecular hydrogen gas which could be introduced into the sputtering atmosphere was accordingly limited.
The difficulty encountered heretofore in reducing the density of localized defect states in the energy gap of amorphous semiconductor materials like silicon and others to desirably low levels, so that these materials are more nearly equivalent of corresponding crystalline materials, is believed to be explainable in the following manner. At or near the Fermi level of these materials deposited, for example, by the glow discharge of silane, are two bumps of relatively high density states in the energy gap which are apparently related to the remaining dangling bond density. They are located substantially at about 0.4 eV below the conduction band E.sub.c and above the valence band E.sub.v. When the glow discharge amorphous silicon is doped with phosphorus or boron, the Fermi level is believed to be moved up or down, but the density of localized states was so high that the dopant could not move the Fermi level close enough to the conduction or valence bands to have an effective p or n junction. Thus, the activation energy for the doped glow discharge amorphous silicon was not lowered below about 0.2 eV. This result also placed a theoretical limitation on the open-circuit photovoltage of a p-n junction of doped glow discharge amorphous silicon, since the internal field cannot exceed the separation of the Fermi level in the p and n type regions. In addition, the remaining activation energy limits the room-temperature DC conduction of the doped glow discharge amorphous silicon and the material would have a large sheet resistance if it were made into a large area array, the resistance not being helped by the rather low carrier mobility which is a factor of about 10.sup.4 -10.sup.5 less than that for crystalline silicon. Also, where it is desirable to modify an amorphous silicon film to form an effective ohmic interface, for example, between an intrinsic (undoped) portion thereof and an outer metal electrode, such modified portions of the film must have a very high conductivity. The prior methods of doping such films do not provide as useful an ohmic interface as in the case of the films of the invention to be described.
The silane glow discharge deposition of silicon films has some limitations in addition to the less than ideal reduction of the density of localized defect states in the energy gap achieved thereby. For example, such a process requires silane, which is a relatively expensive starting material. Also, the structure of such a film and electrical characteristics can vary with the amount of photon absorption therein, and the film is soft and easily scratched or otherwise physically damaged.
The present invention provides an amorphous semiconductor material, like silicon, with a much lower minimum density of localized defect states than was heretofore obtained, to provide more efficient photoconductive and photovoltaic devices. Thus, by providing a much lower density of defect states, the present invention increases carrier lifetime and depletion layer thickness. Also, it enables the amorphous silicon and other materials to be more efficiently modified by the addition of modifying materials thereto.
There has recently issued U.S. Pat. No. 4,196,438 to D. E. Carlson. This patent appears to have as its objective the making of amorphous silicon bodies by glow discharge in a gas atmosphere much less expensive than silane. The patent states that this less expensive gas includes the elements silicon and a halogen selected from the group consisting of chlorine, bromine and iodine. Examples of the deposition gas given are dichlorosilane (SiH.sub.2 Cl.sub.2), chlorosilane (SiH.sub.3 Cl), trichlorosilane (SiHCl.sub.3), bromosilane (SiH.sub.3 Br), dibromosilane (SiH.sub.2 Br.sub.2) and silicon tetrachloride (SiCl.sub.4) with particular emphasis placed on dichlorosilane. Notably and importantly, no reference is made to fluorine. While a comparison is made in this patent between the characteristics of the amorphous silicon devices disclosed therein and the prior sputtered amorphous silicon devices, just as comparisons are made in the prior Carlson U.S. Pat. No. 4,064,521 between the prior amorphous silicon devices fabricated by glow discharge in silane and amorphous silicon devices fabricated by sputtering, there is no reference in U.S. Pat. No. 4,196,438 to any reduced density of localized defect states over that achieved by the use of silane, as is achieved by the present invention. The reduction of this crucial parameter is one of the most important contributions of the present invention.